1. Technical Field
The present invention relates to a solid state laser device generating laser light stably and effectively with a simple and inexpensive structure.
2. Related Art
FIG. 26 is a schematic view showing a structure of the solid state laser device disclosed in Koechner, “Solid-state Laser Engineering”, pp. 342 to 347 (published by Springer Verlag), for example, in the prior art. In FIG. 26, 1 is a solid state laser element containing an active medium, 2 is a semiconductor laser as an excitation light source, 3 is an excitation light emitted from the semiconductor laser array, 4 is a beam shaping optical system for converging the excitation light 3, 5 is a solid state laser light emitted from the solid state laser element 1, and 8 is a partial reflection mirror for transmitting a part of the solid state laser light.
Also, 151 is a side of the solid state laser element 1 directed to the semiconductor laser 2 side, on which a dichroic (two-wavelength) coating that exhibits a low reflectance to the semiconductor laser wavelength and also exhibits a high reflectance to the solid state laser wavelength is applied. Also, 152 is a side of the solid state laser element 1 that is on the opposite side to the semiconductor laser 2, on which the two-wavelength coating that exhibits the high reflectance to the semiconductor laser wavelength and also exhibits the low reflectance to the solid state laser wavelength is applied.
Next, an operation will be explained hereunder. The semiconductor laser 2 serving as the excitation light source generates the excitation light 3 having a wavelength that coincides with the absorption band of the solid state laser element 1. The excitation light 3 emitted from the semiconductor laser 2 is shaped into a desired beam profile by the beam shaping optical system 4, that consists of two lenses, at the side 151 of the solid state laser element 1, and then irradiates the solid state laser element 1. A part of the excitation light 3 that is incident on the solid state laser element 1 excites the active medium contained in the solid state laser element 1 to form a population inversion in the solid state laser element 1.
Also, the excitation light 3 that reaches another side 152 without the absorption in the solid state laser element 1 is reflected toward the side 151 by the dichroic coating applied to the side 152, and then contributes again to the excitation of the active medium contained in the solid state laser element 1.
The high reflection coating and the partial reflection coating that corresponds to the energy difference in the population inversion in the solid state laser element 1 are applied to the side 151 of the solid state laser element 1 and the partial reflection mirror 8, respectively. Both are part of the optical resonator.
A part of the spontaneous emission light generated in the solid state laser element 1 is confined in the optical resonator that consists of the side 151 of the solid state laser element 1 and the partial reflection mirror 8, and goes to and from the inside of the optical resonator. When the spontaneous emission light that travels reciprocally in the optical resonator passes through the population inversion region, the spontaneous emission light is subjected to the amplifying action by the stimulated emission and thus the light intensity in the optical resonator is promptly increased. The coherent solid state laser light 5 increases with an increase of the light intensity, resulting in laser oscillation. The solid state laser light 5 in the optical resonator is extracted to the outside of the optical resonator at a rate that corresponds to the transmittance of the partial reflection mirror 8.